1. Field of the Invention
This invention relates to a method and apparatus for determining the rotational speed of a rotating member, such as a rotary shaft.
2. Description of the Prior Art
Both analog and digital techniques exist for determining the rotational speed of a rotary shaft, such as the shaft of a speed-regulated machine.
Analog rotational speed determination is most often done using a tachogenerator, the electrical output signal of which is only approximately linearly dependent on the rotational speed of the shaft. The output signal is affected by fluctuations caused, for example, by the lamination or the potentials at the carbon contacts. At low rotational speeds, these fluctuations mask the approximately speed-proportional component of the output signal in such a way that exact speed determination is not possible. In addition, there is a reversing error and a long-term drift, caused, for example, by the temperature response of the generator and which also exists at high speeds of rotation.
Digital rotational speed determination is done using an incremental tachometer. Such a device typically comprises a pulse-generating disk mounted for rotation with the shaft. Marks are provided spaced at equidistant discrete angular positions on the periphery of the disk and a stationary probe is positioned adjacent the disk to scan the marks as the disk rotates with the shaft. The marks may be magnetic marks scanned by Hall-effect probes or optical marks scanned by optical sensors.
A periodic electrical signal is generated when the marks rotate past the probe. The probe produces one cycle of the periodic signal in response to the detection of each mark. The number of marks which rotate past the probe (i.e. the number of discrete angular positions traversed as detected from the electrical signal) during a specified time interval determines the rotational speed of the shaft.
The output signal of the probe, as a function of the angle of rotation, is a continuous periodic analog signal which varies steadily between two extreme values. One extreme value corresponds to an angular position of the disk for which the probe is aligned directly opposite one of the marks. The other extreme value corresponds to a disk position for which the probe is opposite a point on the disk periphery which is centered between two marks. The signal generated by the probe is generally sinusoidal or triangular in shape. The number of marks rotating past the probe can be determined by detecting the number of occurrences of a particular event in the probe output signal. This can be done using a threshold detector or comparator to determine the intersection of the output signal with a fixed reference signal. The number of intersections detected during a specified measurement period corresponds to the number of marks that have moved past the probe, i.e. the number of discrete angular positions which lie within the total angular displacement traversed during the measurement period. The total angular displacement of the shaft is equal to the product of this number and the angular increment between marks. For a fixed measurement time period, the rotational speed is thus directly proportional to the number of discrete angular increments swept.
Determination of the rotational speed in a digital manner by counting the integral number of discrete angular positions traversed by the shaft during a specified time period is relatively accurate only if during this period the shaft rotates through a sufficiently large number of discrete angular increments, i.e. for high speeds or long measurement times. For rotational speed determinations at either low speeds or short measurement times, for which total angular rotation of the shaft cannot be accurately approximated by rotation through an integral number of discrete angular increments, such a measurement technique is unsatisfactory.
For this reason some conventional devices combine the analog and digital techniques, by coupling to the rotary shaft both an analog tachogenerator and a digital tachometer, and using the output signal of the digital tachometer to correct the analog rotational speed determination. This duplication of instrumentation is, however, costly. Further, for mechanical and operational safety reasons, the use of an analog tachogenerator may be undesirable in many applications. Notwithstanding this, exact determination of low rotational speeds is still not possible. Moreover, such combination devices cannot determine a zero rotational speed, such as occurs during machine standstill or upon reversal of the direction of rotation.